The present invention relates to a method of forming a polycide gate of a semiconductor device, and more particularly, to an etching method for forming such a polycide gate.
Among semiconductor devices, in a metal oxide semiconductor (MOS) type of a semiconductor device, a gate is formed on a thin gate oxide film. At this time, the gate is formed of a conductor such as a polysilicon, a polycide or the like. The polycide is formed as a stacked structure in which a portion coming into contact with the gate oxide film consists of a polysilicon layer and a refractory metal silicide layer stacked on the polysilicon layer. As the refractory metal silicide, tungsten silicide (WSi) is mainly used.
As the scale of integration of semiconductor devices is increased and the critical dimension is decreased, the resistance of the polysilicon mainly used hitherto is increased, so that its function as a conductor causes a problem. At the same time, metal silicide is suitable to be used as a conductor such as for the gate of a semiconductor device, because damage caused due to the high temperature of its production process is small and its conductivity is high. However, since its adhesion to an insulating film is poor, the metal polycide structure is formed by using a method where the polysilicon is formed on the insulating film and the metal silicide is stacked thereon.
In a conventional method of forming the gate, as a material constituting the gate, a film is formed on the entire surface or a part of a semiconductor substrate including a portion where the gate will be formed. After forming an etching mask by using photolithography, a gate film which is in a part excluding the gate is removed by using an etching technique. At this time, as a method of forming the etching mask by using photolithography, there are two methods. One method is that a film is formed of a material constituting the gate, a photoresist is directly formed thereon, and a pattern is formed in the photoresist layer, thereby adapting the photoresist to function as an etching mask. The other method is that a film is formed of a material constituting the gate, an oxide film is formed thereon, a photoresist is formed thereon, a gate pattern is formed in the oxide film by etching the photoresist and oxide film, and the gate film is etched by using the above referenced pattern. The two methods are selectively used according to the necessity of the process, because they have respective merits and demerits.
In case of using the oxide film as the etching mask, the etching selectivity between the etching mask and film to be etched is improved. In case of performing the etching process by using the photoresist pattern, there is a merit that a problem of controlling the critical dimension is removed, due to a polymer which adheres to the sidewall of the pattern as by-product. Thus, it is suitable for the manufacturing of highly integrated semiconductor devices.
FIG. 1 shows a polycide gate structure formed through the conventional etching process, using a silicon oxide film as an etching mask 11, and a tungsten silicide 15 as the metal silicide. A gate oxide film 17, or a field oxide film 20, is formed on a semiconductor substrate 10. A polycide gate composed of a polysilicon layer 13 and a tungsten silicide layer 15, and an etching mask 11 consisting of a silicon oxide film, constitute the stacked structure.
Since no protection for the etching is provided in the sidewall part of the polycide gate, in case of using the conventional etching technique such as plasma etching or reactive ion etching (RIE), the undercutting occurs in the polysilicon layer 13, due to the characteristics of isotropic etching. The sidewall of the stacked structure constituting the polycide becomes rough due to particles such as ions or radicals which collide with the sidewall of the polycide gate. There is seriously generated a phenomenon such as a consuming or notching, in which the films of the metal silicide and polysilicon are removed during etching. Thus, it is difficult to control the uniformity of the semiconductor device, so that the function of the device is deteriorated. Reference numeral 19 in FIG. 1 shows the consumption of tungsten silicide 15.
FIG. 2 is an enlarged fragmentary cross-section of a detail of the structure shown in FIG. 1, showing the roughness and defect of the polycide gate sidewall. The undercut part 23 of the polysilicon layer 13 and the consumed part 25 of the metal silicide layer 15 are shown.
The polycide gate structure consists of the films of metal silicide and polysilicon, and the layer located thereunder is composed of the thin oxide film. Accordingly, through a single etching process under constant conditions, it is very difficult to form a gate at a proper etching speed, sufficient etching selectivity for the etching mask and a plurality of material layers located thereunder, and a sidewall which has few defects and maintains good vertical morphology.
As another conventional method for solving such problems, there was used a method in which the etching is performed under the process condition beyond the above described two kinds, in which the conditions such as the plasma source gas for carrying out the etching process, the power of the equipment applying the plasma, and the pressure and temperature of the process chamber are changed. However, in case of performing the process with two kinds of steps or more, even if the process conditions of each step are changed minutely, the result of the process shows a very large difference. The aforementioned defect of the gate sidewall or the poor vertical profile shows a difference in one semiconductor substrate as well as between substrates. It was very difficult to maintain the stability of the process.